In recent years, with the rapid advancement in the infrastructural network of information and communication, increase in power consumption has become a social issue. Power supply system for communication apparatuses, in particular, is shifting from centralized supply to decentralized supply in order to meet demands for reduction in size of the equipment and power consumption therein. Today, for such power units, small and thin onboard power supplies are being widely used. On the other hand, to meet the demands for large current required for speedup of LSI and for reduction of power consumption, a low-voltage setup is being rapidly advanced. Measures that meet demands for lower voltage and larger current a required of onboard power units for driving such LSIs. There is a technological tendency toward increasing the switching frequency as a measure to achieve a further reduction in size of the thin onboard power unit. Especially for the transformer as the major component of the power supply unit, there is a demand for a thin transformer of a surface-mount type that is suited for high-frequency driving, has low-loss and low-noise characteristics, small in size, and low in price.
To meet the need for development of such power units, a laminated-coil thin transformer is disclosed in Japanese Patent Laid-open Application No. H10-340819. A coil base is used therein for positioning coils that are piled up. Also, there is an attempt not to use a positioning coil base for increasing the space factor of the coil, thereby enhancing the electrical characteristic of the transformer. FIG. 10 is an exploded perspective view of a conventional multilayered thin transformer having no coil base for positioning of coils to be piled up. FIG. 11 is a sectional view showing the multilayer structure of the conventional multilayered thin transformer of FIG. 10. Two each of non-wirewound primary coils and secondary coils are produced from a conductor in a thin plate form by such a method as punching or etching. A multilayered coil assembly is fabricated by piling insulating paper 3, secondary coil 2, insulating paper 3, primary coil 1, insulating paper 3, secondary coil 2, insulating paper 3, primary coil 1, and insulating paper 3, one on another, as shown in FIG. 10. Then, a suitable amount of adhesive 8, for bonding magnetic core 5 to the multilayered coil, is applied to the top and bottom faces of the multilayered coil. Finally, magnetic cores 5 are mounted in place from above and below and, thereby, a thin transformer is completed. After the completion of the transformer, each coil is connected with a terminal. Each coil is connected to terminal 6 provided on main-unit base 9 via connection portion 7 by such a method as soldering or welding as shown in FIG. 11. In the conventional example shown in FIG. 10, coils are piled up without using a coil base for positioning the coils.
Therefore, relative positions between coils and insulating paper 3 become unstable. Hence, as shown in FIG. 11, great variations are produced in distance A between a primary coil and a secondary coil and distance B between a coil and a magnetic core.
Further, since the coils are piled up individually, operability in the mounting of the magnetic core is much impaired. As a result, insulation performance and electrical performance are not stabilized and hence great problems in terms of quality and productivity arise.
The present invention aims to solve the above discussed problems in the conventional art examples and to provide a multilayered thin transformer of a coil-baseless type providing stabilized insulating performance and electrical performance and manufactured with high productivity, as well as to provide a method of manufacturing the same.